1. Field
The disclosure relates generally to the field of Raman spectroscopy. More specifically, the disclosure relates to preparation of surface-enhanced Raman scattering (SERS) substrates derivatized to have increased lifetime and affinity for analytes of interest. Such compositions and methods have use in the areas of homeland security and force protection, for example, in the detection of trace samples including BTEX (benzene, toluene, ethylbenzene, and xylenes), chlorinated solvents, TNT, nerve agents, blister agents, metal ions, anions, antigens, peptides, nucleic acids, spores, fungi, viruses, and bacteria.
2. Background of the Subject Matter
The present disclosure generally relates to the field of Raman spectroscopy, and more particularly, to the preparation of surface-enhanced Raman scattering (SERS) substrates derivatized to have increased lifetime and affinity for analytes of interest.
Raman spectroscopy is an emission technique that involves inelastic scattering of incident laser energy and results in spectral peaks that are frequency shifted from the incident energy. The Raman bands arise from changes in polarizability in a molecule during vibration. As a result, virtually all organic molecules display a characteristic Raman emission. Therefore, a Raman sensor would not be limited to a specific class of molecules as is the case for the laser induced fluorescence (LIF) sensor. Raman spectrometry allows the fingerprinting of species present and is structurally specific. The inherently high resolution of Raman spectra often permits the analysis of several components in a mixture simultaneously.
Despite the advantages of Raman spectroscopy over other spectroscopic techniques and the technological advances in the area of Raman spectrometry, Raman spectroscopy is, inherently, an insensitive technique. To achieve detection limits in the low ppm range would require either the use of a multiple pass cell or long acquisition times. In the 1970s, it was discovered that Raman scattering from molecules adsorbed on such noble metals as silver, copper, and gold can be enhanced by as much as 106 to 107 (Fleischmann et al., Chem. Phys. Lett., 26:163 (1974); Jeanmaire et al., J. Electroanal. Chem., 84:1 (1977).) This phenomenon, called surface enhanced Raman spectroscopy (SERS), is still not understood despite intensive theoretical and experimental research. It is believed that more than one mechanism is involved in the SERS phenomenon. Initially, the SERS technique was used as a means to probe adsorption at metal interfaces both in electrochemical and gas-phase environments. This technique has proven useful in deducing the effects of interfacial structure and reactivity on the adsorption process, and the sensitivity of the technique as well as its exceptional spectral selectivity has made SERS attractive for a broad range of analytical applications. Garrell, Anal. Chem., 61:401A (1989). SERS can be used for trace organic analysis and as a detection method in gas chromatography, liquid chromatography, and thin layer chromatography. Electrochemical SERS (Carrabba, et al., Fiber Optic Raman Chemical Sensors, in Proceedings of the Symposium on Chemical Sensors A. Riccio and N. Yammazoe, Eds., The Electrochemical Society, Pennington, N.J., p. 634 (1993); Storey, et al., Appl. Spectrosc., 48:1265 (1994)) and SERS of chemically modified surfaces (Canon, et al., Environ. Sci. Technol., 26:1950 (1992); Mullen et al., Anal. Chem., 66:478 (1994)) may be used to detect aromatic compounds and chlorinated hydrocarbons, organic contaminants of environmental concern, in the ppm concentration range.
The SERS technique requires intimate contact between the SERS active surface and analyte. In turn, this requires that the analyte bond to the SERS active surface. Current SERS substrate configurations allow detection limits in the upper ppb to low ppm concentration range. However, there may be requirements for detection limits in the low ppb to high pptr concentration range. Further, SERS substrates readily become poisoned by chemical reactions and result in decreased sensitivity and reliability for detecting Raman scattering due to excitation of the analyte of interest.
Therefore, a need exists for a SERS substrate that minimizes poisoning by chemical reactions and/or increases the lifetime of a SERS substrate's sensitivity and reliability. A means of preparing robust SERS substrates that could be used to detect analytes in the pptr-ppb concentration range would result in a sensing capability that could be used to monitor analytes in real time and in-situ. Besides environmental monitoring, such a sensor could be used for homeland security and force protection. There is a real concern that terrorists could poison water supplies using readily available toxic industrial chemicals such as the BTEX compounds, chlorinated solvents, and anions such as nitrate and perchlorate.